1. Field of the Invention
This invention relates to a method for recovering petroleum from subterranean petroleum containing formations and more particularly to a surfactant flooding method for recovering petroleum from petroleum containing formations. Still more specifically, this invention pertains to a novel surfactant mixture which will function effectively in the presence of formation waters containing high polyvalent ion concentrations, e.g., high calcium or magnesium concentrations. which cause precipitation of conventional surfactants, and to a method for using such novel surfactant compositions in a surfactant flooding process for recovering petroleum.
2. Description of the Prior Art
Petroleum is normally recovered from subterranean formations in which it has accumulated by penetrating said formations with one or more wells and pumping or permitting the petroleum to flow to the surface through these wells. Recovery of petroleum from such petroleum containing formations is possible only if certain conditions are satisfied. There must be adequately high concentration of petroleum in the formation, and there must be sufficient porosity and permeability or an interconnected flow channel throughout the formation to permit the flow of fluids therethrough if sufficient pressure is applied to the fluid. When the subterranean petroleum containing formation has natural energy present in the form of an underlying active water drive or gas dissolved in the petroleum which can exert sufficient pressure to drive the petroleum to the producing well or a high pressure gas cap above the petroleum within the petroleum reservoir, this natural energy is utilized to recover petroleum. Recovery of petroleum by utilization of natural energy is referred to as primary recovery. When this natural energy source is depleted in the instance of those formations which do not originally contain sufficient natural energy to permit primary recovery operations, some form of supplemental recovery process must be utilized in order to extract petroleum from the subterranean petroleum containing formation. Supplemental recovery is frequently referred to as secondary recovery, although in fact it may be primary, secondary or tertiary in sequence of employment.
Water flooding which involves the injection of water into the subterranean petroliferous formation for the purpose of displacing petroleum for the producing well is the most economical and widely practiced supplemental recovery method. Water does not displace petroleum with high efficiency however, as water and oil are immiscible and also because the interfacial tension between water and oil is quite high. Persons skilled in the art of oil recovery have recognized this weakness of waterflooding and many additives have been described in the prior art for decreasing the interfacial tension between the injected water and the formation petroleum. For example, U.S. Pat. No. 2,233,381 (1941) disclosed the use of polyglycol ether as a surface active agent or surfactant to increase the capillary displacement efficiency of an aqueous flooding medium. U.S. Pat. No. 3,302,713 discloses the use of petroleum sulfonate prepared from the 850.degree. to 1,050.degree. F. boiling range fraction of petroleum crude as a surfactant for use in oil recovery operations. U.S. Pat. No. 3,468,377 (1969) describes the use of petroleum sulfonates of a specified molecular weight for oil recovery. Other surfactants which have been proposed for oil recovery include alkyl sulfates, alkyl sulfonates, and quaternary ammonium salts.
The above described surfactants are satisfactory for surfactant flooding in petroliferous formations only if the calcium and magnesium concentration of the formation water is low, generally well below about 500 parts per million. Petroleum sulfonate is one of the most popular and desirable surfactants because of its high surface activity and low unit cost, although it suffers from the limitation that it can only be used when the total formation water hardness (calcium plus magnesium) is less than about 500 parts per million. If the formation water calcium and/or magnesium content exceeds about 500 parts per million, petroleum sulfonates precipitate rapidly. When precipitation of the added material occurs, not only is the desired beneficial result lost, but plugging of the formation will very likely result.
Many subterranean petroleum containing formations are known to exist which contain polyvalent ions such as magnesium and calcium in concentrations far in excess of 500 parts per million, the most common of such reservoirs are limestone formations which may have polyvalent ion concentrations from 1,000 to as high as 20,000 parts per million in the original connate water, and the formation water after the formation has been subject to flooding with fresh water may have concentrations of calcium and/or magnesium from about 500 to about 15,000 parts per million. Since surfactants usable for oil recovery operations precipitate when exposed to aqueous environments having a total hardness in excess of about 500 parts per million, such surfactants could not be used in limestone reservoirs. If an aqueous solution of petroleum sulfonates, for example, were injected into a limestone reservoir, the petroleum sulfonate would precipitate immediately on contacting the high calcium containing formation water. In such a process, the flood water would have essentially no surfactant present in it to decrease the interfacial tension between the water and the petroleum. Furthermore, precipitated petroleum sulfonate plugs the small flow channels in the subterranean petroleum containing formation, decreasing the formation porosity and injectivity, thereby causing a substantial decrease in the oil displacement efficiency.
Nonionic surfactants, such as polyethoxylated alkyl phenols, polyethoxylated aliphatic alcohols, carboxylic esters, carboxylic amides, and polyoxyethylene fatty acid amides, have a somewhat higher tolerance of polyvalent ions such as calcium or magnesium than do the more commonly utilized anionic surfactants. While it is technically feasible to employ a nonionic surfactant solution to decrease the interfacial tension between the injected aqueous displacing medium and petroleum contained in some limestone formations, such use would not economically feasible for several reasons. Nonionic surfactants are not as effective on a per unit weight basis as are the more commonly used anionic surfactants, and furthermore, the nonionic surfactants have a higher cost per unit weight than do the anionic surfactants.
Thus, it can be seen that while many surfactants have been proposed for supplemental oil recovery use, there is a substantial, unfilled need for a surfactant system usable in the presence of formation waters containing calcium and/or magnesium in excess of 500 parts per million.